Ophthalmic instruments are used, for example, to correct an ametropia of the human eye by means of a surgical laser operation on the cornea. A method of particular importance is known as “LASIK” (laser in-situ keratomileusis), in which, by application of a pulsed laser beam, material is ablated from inside the cornea rather than from its surface. For this purpose, a flap is formed on the outer corneal surface, the thickness of this flap being substantially smaller than the thickness of the cornea. For the ablation treatment proper, the flap is folded back, whereupon a pulsed laser beam is used to ablate material from the exposed area to correct the ametropia. Thereafter, the flap is closed again onto the treated surface.
To form the flap at a precisely defined depth of the cornea and as gently and precisely as possible, it has been suggested to use femtosecond laser pulses, i.e. laser pulses having pulse widths smaller than 10−12 s. By application of such pulses it is possible to create localized optical ruptures in the cornea, known as photodisruptions, the size of which is limited to a few micrometers. By positioning a dense array of many such disruptions in exactly defined places, the flap can be made to measure very precisely. For the precise forming of the flap it is essential that the focus of the pulsed laser beam is exactly positioned not only laterally but also, and mainly so, in the depth of the cornea, i.e. in the propagation direction of the laser beam.
In this connection, DE 10 2005 013 949 A1 describes a scanning device for, and a method of, focusing a beam of rays into a given volume. This scanning device is especially suitable for use in instruments intended for surgical laser treatment of the cornea with laser radiation pulsed in the femtosecond range. The device allows the laser light to be focused on any target points within the cornea.
DE 10 2008 027 358 A1 describes a laser system that can be used in ophthalmic instruments for the analysis and treatment of the lens of the eye. Other than laser systems in comparable instruments, it has the advantage that the detection of the laser light backscattered in the lens of the eye is possible with greater accuracy, so that a refractive surgical therapy of the lens of the eye can be performed with higher precision. For this therapy, laser radiation pulsed in the femtosecond range is focused on selected target points within the lens of the eye.
The state of prior art described above regarding the medical examination and surgical laser treatment of the cornea (situated at the periphery of the eye) on the one hand and the lens (situated within the eye) on the other has the disadvantage that the ophthalmic instruments available satisfy the requirements of one of these special applications only; i.e., they differ with regard to their imaging properties, in particular: to their aperture, focal spot size, focus position in the eye, correction of the aperture aberration or correction of field aberrations to such an extent that they are exclusively suitable either for the examination and treatment of the cornea or for the examination and treatment of the lens of the eye.
This means that an oculist needs to procure two instruments, which is inefficient (a) with regard to acquisition cost and (b) because several single-purpose instruments are seldom used to capacity. In addition, separate instruments involve extra time for getting adjusted at least to the same eye of a patient being examined.